1. Field of the Invention
The present application relates to lost foam casting processes in general and more particularly to a lost foam process for casting low carbon ferrous metals including stainless steel and heat treatment method for same.
2. Description of the Prior Art
Casting processes using lost foam are known and a description of such a process may be found in U.S. Pat. No. 2,830,343 granted to H. F. Shroyer. This casting process utilizes a cavityless casting method wherein a polystyrene foam pattern is embedded in sand. The foam pattern left in the sand is decomposed by molten metal that is poured into the foam pattern. The molten metal replaces the foam pattern thereby precisely duplicating all of the features of the pattern. Similar to investment casting using lost wax, the pattern is destroyed during the pouring process and a new pattern must be produced for every casting made.
The above described process thus utilizes the following basic steps. First a foam pattern and gating system is made using some sort of mold. Secondly, the mold or foam pattern and gating system are usually assembled into a cluster of individual parts to facilitate large volume production. The cluster is then coated with a permeable refractory coating. The prepared cluster is then placed into loose unbonded sand that is packed around the foam cluster by vibrating the entire mold assembly. The molten metal is then poured directly into the foam cluster decomposing the foam in the cluster and replacing it with the poured metal. The cluster is then removed, separated and the individual parts finished off in well known methods.
The previously described lost foam process has been used to produce gray iron and non-ferrous material parts. To-date, stainless steel has been impractical to pour utilizing the above procedure. The stainless steel molten metal generates carbon when it is volatilized and the carbon is absorbed into the liquid metal thereby raising the carbon level of the finished stainless steel product. Certain applications for stainless steel have ASTM Standards for carbon content that are within the ranges of 0.06% to 0.08% carbon. One such application for such stainless steel parts that have to be made according to this ASTM Standard is the tube hangers for nuclear reactors which require the parts to be produced from ASTM grade material A297HH. Low carbon ferrous metals may be defined as having a carbon content of 0.03% to 0.5% by weight.
Attempts were made to manufacture these mentioned tube hangers from stainless steel according to the above described lost foam process with unsatisfactory results. The sand surrounding the foam forms was even subjected to vacuums between 4" and 12" of mercury applied to the flask holding the sand and the parts to maintain the sand around the part. Even with these mentioned vacuum ranges, which are suggested in the prior art to maintain process integrity, the results were unsatisfactory.
Thus, it is seen that a lost foam process for manufacturing stainless steel parts which require a low carbon level according to application standards, such as those set by ASTM, was a requirement that was not met by the prior art. Also, conventional method for heat treating castings is to place the castings into containers without regard to required spacing for controlled heating and cooling of these parts. Thus, a need still exists for a heat treating method that provides proper spacing of parts to be heat treated.